Carbon dioxide gas sensors and method of manufacturing and using same

ABSTRACT

A gas sensor includes a substrate and a pair of interdigitated metal electrodes selected from the group consisting of Pt, Pd, Au, Ir, Ag, Ru, Rh, In, and Os. The electrodes each include an upper surface. A first solid electrolyte resides between the interdigitated electrodes and partially engages the upper surfaces of the electrodes. The first solid electrolyte is selected from the group consisting of NASICON, LISICON, KSICON, and β″-Alumina (beta prime-prime alumina in which when prepared as an electrolyte is complexed with a mobile ion selected from the group consisting of Na + , K + , Li + , Ag + , H + , Pb 2+ , Sr 2+  or Ba 2+ ). A second electrolyte partially engages the upper surfaces of the electrodes and engages the first solid electrolyte in at least one point. The second electrolyte is selected from the group of compounds consisting of Na + , K + , Li + , Ag + , H + , Pb 2+ , Sr 2+  or Ba 2+  ions or combinations thereof.

ORIGIN OF THE INVENTION

The invention described herein was made by employees and by employees ofa contractor of the United States Government, and may be manufacturedand used by the government for government purposes without the paymentof any royalties therein and therefor.

FIELD OF THE INVENTION

The invention is in the field of carbon dioxide gas sensors.

BACKGROUND OF THE INVENTION

The detection of CO₂ is essential for a range of applications includingreduction of false fire alarms, environmental monitoring, and engineemission monitoring. For example, traditional smoke detectors monitoringparticles can have false fire alarm rates as high as 1 in 200 inaircraft applications. Alternatively, monitoring the change of CO andCO₂ concentrations and their ratio (CO/CO₂) can be used to detect thechemical signature of a fire. Electrochemical CO₂ sensors which usesuper ion conductors (such as Na Super Ionic Conductor or NASICON) asthe solid electrolyte, and auxiliary electrolytes (such as Na₂CO₃/BaCO₃)have great potential for in-situ fire detection and other applications.In recent years, there has been a significant effort to develop bulk andminiaturized electrochemical CO₂ sensors. Compared to bulk material andthick film solid electrolyte CO₂ sensors, miniaturized sensorsfabricated by microfabrication techniques generally have the advantagesof small size, light weight, low power consumption, and batchfabrication.

Four factors are typically cited as relevant in determining whether achemical sensor can meet the needs of an application, namely,sensitivity, selectivity, response time and stability. Sensitivityrefers to the ability of the sensor to detect the desired chemicalspecies in the range of interest. Selectivity refers to the ability ofthe sensor to detect the species of interest in the presence ofinterfering gases which also can produce a sensor response. Responsetime refers to the time it takes for the sensor to provide a meaningfulsignal. By meaningful signal it is meant that the signal has reached,for example, 90% of the steady state signal when the chemicalenvironment experiences a step change. Stability refers to the degreewhich the sensor baseline and response are the same over time. It isdesirable to use a sensor that will accurately determine the species ofinterest in a given environment with a response large and rapid enoughto be of use in the application and whose response does notsignificantly drift over its operational lifetime.

Current bulk or thick film solid electrolyte carbon dioxide sensors havethe disadvantages of being large in size, high in power consumption,difficult in batch fabrication, and high in cost. The carbon dioxidesensor design described herein has the advantage of being simple tobatch fabricate, small in size, low in power consumption, easy to use,and fast responding.

FIG. 1A is a cross-sectional schematic illustration 100 of a prior artbulk carbon dioxide gas sensor. Referring to FIG. 1A, reference numeral101 is an electrolyte known as NASICON which is an acronym or partialacronym for Na₃Zr₂Si₂PO₁₂ and is oriented between a platinum (paste) 103and a Sodium Carbonate/Barium Carbonate (Na₂CO₃/BaCO₃) layer 102. Areference electrode 105 engages the platinum paste and a gold workingelectrode 104 resides in contact with the interface of the SodiumCarbonate and/or Barium Carbonate (Na₂CO₃/BaCO₃) 102 and the NASICON101. By Sodium Carbonate and/or Barium Carbonate (Na₂CO₃/BaCO₃), it ismeant that either Sodium Carbonate (Na₂CO₃) or Barium Carbonate (BaCO₃),or their mixtures may be used. The sensor is supported by quartz glasstubes (insulators) 106 for reference gases.

FIG. 1 is a cross-sectional schematic illustration 100 of a prior artgas sensor disclosing an Alumina substrate 107, interdigitated Platinummetal electrodes 108, a first solid electrolyte, NASICON 109, betweenthe electrodes, and Sodium Carbonate and/or Barium Carbonate(Na₂CO₃/BaCO₃) 110 covering the NASICON and the electrodes. The firstsolid electrolyte is selected from the group consisting of NASICON,LISICON, KSICON, and β″-Alumina (beta prime-prime alumina in which whenprepared as an electrolyte is complexed with a mobile ion selected fromthe group consisting of Na⁺, K⁺, Li⁺, Ag⁺, H⁺, Pb²⁺, Sr²⁺ or Ba²⁺). BySodium Carbonate and/or Barium Carbonate (Na₂CO₃/BaCO₃), it is meantthat either a material containing Sodium Carbonate (Na₂CO₃), BariumCarbonate (BaCO₃), or a mixture of Sodium Carbonate and Barium Carbonatemay be used. An important feature of electrochemical cells of this typeare the three-contact boundaries seen in 100. It is the intersection of108, 109, and 110. These contacts significantly determine theeffectiveness of the sensor and their number and surface area should bemaximized. The inventors of the instant patent application disclosedthis structure in a conference in Lisbon, Portugal in 2004 and thisstructure was illustrated or described in an FAA website thereafter.This structure is a schematic and not ideally achievable for a number ofreasons. First, to obtain the structure exactly as illustrated in FIG. 1a perfectly sized and aligned mask is necessary. In other words thewidth of the mask and its apertures has to be absolutely perfect and thealignment has to be absolutely perfect to achieve uniform three-pointcontact along the joint of the metal electrodes, NASICON and SodiumCarbonate/Barium Carbonate (Na₂CO₃/BaCO₃). Statistically, givenmanufacturing tolerances the structure depicted in FIG. 1 is verydifficult to achieve. Photolithographic masks are aligned by hand withthe aid of an electron microscope. Any misalignment of thephotolithographic mask will result in photoresist trapped betweenNASICON and electrode finger and therefore result in a failed sensor.Simply put, the structure of FIG. 1 is very difficult to manufactureexactly as shown. Errors in manufacturing probably will result in afailed structure such as that depicted in FIG. 5D. One of theinnovations of the instant invention is to realize the advantages of nothaving to perfectly duplicate the structure of FIG. 1, which representsthe structure obtained using standard procedures of microfabricationengineers.

Previously, most solid electrolyte CO₂ sensors developed were bulk sizedor thick film based as illustrated in FIG. 1A, which involvescomplicated fabrication process of hot press or screen printing. Thepower consumption of these sensors is very high and batch fabrication isvery difficult. Porous electrodes are typical: Electrodes formed by thethick film technique are not sufficiently porous. Using a non-porouselectrode can lead to the formation of sodium carbonate Na₂CO₃ whichhinders the working electrode. The formation and dissociation of sodiumcarbonate Na₂CO₃ at the electrodes results in slower response time.

Most often (in the prior art) two sensing materials were used in a solidelectrolyte CO₂ sensor structure. In the effort to miniaturize a CO₂sensor, the standard approach was to first deposit one sensingelectrolyte on the substrate, the electrodes were then deposited on topof the electrolyte, and finally the auxiliary electrolyte was depositedon the electrodes. Humidity, liquid chemical processing, and/or physicalvibration tends to erode or loosen the electrolyte underneath theelectrodes. This structure limited the application of standardmicroprocessing techniques one might employ such as photolithography.These properties limited the miniaturization of the sensor using thisstructure, because the electrodes could only be deposited by a shadowmask, which usually produces electrodes with less integrity when thefeature is very small. That is one reason few stable and functionalminiaturized sensors of this type exist.

Photolithography is used in device fabrication processes every time apattern is transferred to a surface. It allows ion implantation oretching of a material in selected areas on the wafer (substrate).Photoresist is a photosensitive organic substance which is a stickyliquid with high viscosity which is typically spun onto a wafer and thenthermally hardened in an oven. Photoresist may be positive or negative.When positive photoresist is exposed to light it breaks down long-chainorganic molecules into shorter chain molecules which can be dissolved bya chemical solution called a developer. When negative photoresist isexposed to light it induces cross-linking of organic molecules such thata high atomic mass is achieved by producing longer-chain molecules. Inthe example of longer chain molecules, an appropriate developer solutionis then used to remove the resist that has not been exposed to light.The transfer of the desired patterns onto the photoresist is made usingultraviolet light exposure through a mask which is typically a quartzplate. Masks are used in two modes. Contact lithography involvesoverlaying the mask directly into contact with the photoresist andproximity photolithography involves spacing the mask a distance abovethe photoresist. The use of photolithography enables miniaturization,batch processing, and more exact duplication of a given sensorstructure. Employing these techniques can fundamentally change andimprove the sensors produced; a significant technical challenge is toapply these techniques for some material systems such as those used forCO2 sensor production.

SUMMARY OF THE INVENTION

A miniaturized amperometric electrochemical (solid electrolyte) carbondioxide (CO₂) sensor using a novel and robust sensor design has beendeveloped and demonstrated. Semiconductor microfabrication techniqueswere used in the sensor fabrication and the sensor is fabricated forrobust operation in a range of environments. The sensing area of thesensor is approximately 1.0 mm×1.1 mm. The sensor is operated byapplying voltage across the electrodes and measuring the resultantcurrent flow at temperatures from 450 to 600° C. Given that air ambientCO₂ concentrations are ˜0.03%, this shows a sensitivity range from belowambient to nearly two orders of magnitude above ambient. Sensor currentoutput versus ln [CO₂ concentration] (natural logarithm of the carbondioxide concentration) shows a linear relationship from 0.02% to 1% CO₂.This linear relationship allows for easy sensor calibration. Linearresponses were achieved for CO₂ concentrations from 1% to 4% and to thelogarithm of the CO₂ concentrations from 0.02% to 1%. These sensingmeasurement results, but not the method of sensor fabrication, weredisclosed in the April 2004 American Ceramic Society presentation and atthe Fire Prevention Conference in Lisbon November 2004. This CO₂ sensorhas the advantage of being simple to batch fabricate, small in size, lowin power consumption, easy to use, and fast response time.

One aspect of the development of the invention was to develop miniatureCO₂ sensors for a wide variety of applications. This miniaturized CO₂sensor can be integrated into a sensor array with other sensors such aselectronics, power, and telemetry on a postage stamp-sized package. Likea postage stamp, the complete system (“lick and stick” technology) couldbe placed at a number of locations to give a full-field view of what ischemically occurring in an environment.

The development of miniature electrochemical sensors based on solidelectrolytes NASICON (Na₃Zr₂Si₂PO₁₂) and Na₂CO₃/BaCO₃ for CO₂ is animportant aspect of the instant invention. Semiconductormicrofabrication techniques are used in the sensor fabrication. Thefabrication process involves three fabrication steps: 1) deposition ofinterdigitated electrodes on alumina substrates; 2) deposition of solidelectrolyte NASICON (Na₃Zr₂Si₂PO₁₂) between the interdigitatedelectrodes; and 3) deposition of auxiliary solid electrolytes Na₂CO₃and/or BaCO₃ (1:1.7 molar ratio) on top of the entire sensing area. Theresulting sensing area is approximately 1.0 mm×1.1 mm. The multipleinterdigitated finger electrodes are in contact with the solidelectrolytes and the atmosphere in multiple locations rather than injust one location as is seen with single set of electrode structures.Thus, this approach yields increased surface area associated withthree-contact boundaries as compared to other sensors with similardimensions. The same sensor structure could also be applied to developother sensors such as NO_(x) sensors with the corresponding auxiliaryelectrolytes NaNO₂ or NaNO₃.

An amperometric circuit is used to detect CO₂. The detection systemincludes pairs of electrodes with constant voltage, V, applied acrossthe electrodes.

The sensing mechanism of the amperometric CO₂ sensors can be understoodbased on the reactions taking place at the working and referenceelectrode of each pair of electrodes. The following two reactions may beconsidered to carry current between the electrodes:Working Electrode 2Na⁺+CO₂+½O₂+2e ⁻→Na₂CO₃Reference Electrode Na₂O→2Na⁺+½O₂+2e ⁻The reduction current is the result of the reaction taking place at theworking electrode where electrons are consumed. The oxidation current isthe result of the reaction taking place at the reference electrode whereelectrons are released.The following reaction can then be considered to be:Overall Reaction Na₂O+CO₂→Na₂CO₃

Platinum is used as the preferred material for the electrode. However,electrodes made from other metals such as Palladium, Silver, Iridium,Gold, Ruthenium, Rhodium, Indium, or Osmium may also be used. Inaddition, non-porous or porous electrodes may be used

The auxiliary electrolyte (Na₂CO₃ and/or BaCO₃) is depositedhomogeneously on the entire sensing area of the sensor, including boththe working and reference electrodes. The deposition of an auxiliarycarbonate electrolyte improves the selectivity and sensitivity of thesensor to CO₂ gases and the flow of the desired species within theelectrolyte. At the working electrode, depleted concentration of sodiumions (Na⁺) can be recovered by the transfer of sodium ions (Na⁺) fromNASICON through the three-phase boundary of the electrodes, NASICONelectrolyte, and an auxiliary electrolyte layer. The sodium carbonate,Na₂CO₃, deposited at the working electrode during reacting with CO₂ canbe transferred to the reference electrode through the Na₂CO₃/BaCO₃auxiliary carbonate electrolyte layer if temperatures are high enough,for example, 450-600° C.

These mechanisms allow the sensor to measure CO₂ but recover back to itsinitial state. The sensing mechanism has increased performance from theNa₂CO₃/BaCO₃ auxiliary carbonate electrolyte layer being distributedacross both the working and the reference electrodes at high operatingtemperatures in the 450-600° C. The eutectic mixture of Na₂CO₃/BaCO₃ asthe auxiliary carbonate electrolyte layer has a lower meltingtemperature enabling improved flow within the electrolyte at a reducedtemperature range. The Na₂CO₃/BaCO₃ auxiliary carbonate electrolyte canact as a diffusion barrier to prevent other species from reaching theelectrode/electrolyte interface and interfering with the correlation ofmeasured current with detection of the desired chemical species.

In order to facilitate a faster response time, porous platinumelectrodes can be used with an auxiliary carbonate electrolyte having anincreased porosity. The sensor structure employs interdigitatedelectrodes which can be generally thought of as interdigitated fingers.Unique fabrication processes to miniaturize the CO₂ sensor are used.

A unique amperometric CO₂ sensor is produced using a non-standardapproach as disclosed herein and has the following attributes:

First is the miniature size of the sensor with interdigitatedelectrodes. The fabrication of electrodes with photolithography enablesthe sensor to have a small sensor sizes with a sensing area ofapproximately 1.0 mm×1.1 mm (electrode width and spacing betweenelectrodes is around 50 μm). Further miniaturization is possible and thesize can be varied to control sensor properties. The sensor would bevery difficult to make with a shadow mask if a layer of electrolyte isdeposited before the electrodes as is the case in most other attemptedprocesses. Interdigitated electrodes are very important for amperometricCO₂ sensors because the current output of the electrodes is summed andbussed which results in currents much higher compared to the traditionaltwo electrodes with the same size. As a result, better sensitivity ofthe sensor is achieved. In other words for a given change of input tothe sensor in terms of CO₂ concentration, a larger differential changein output is observed.

Secondly, the sensor has a robust structure. The interdigitatedelectrodes were deposited directly on the alumina substrate with strongadhesion, which will stand the attack of humidity and vibration. This isin contrast to the approach of depositing the electrolyte first on thesubstrate which has less inherent stability.

Thirdly, the sensor has a unique arrangement of electrodes/electrolytes.Solid electrolyte NASICON is deposited between interdigitated fingersand the auxiliary electrolyte Na₂CO₃/BaCO₃ was deposited on the wholesensing area, forming greater length of three-point boundaries(electrode, solid electrolyte NASICON, and auxiliary electrolyteNa₂CO₃/BaCO₃), which is beneficial for amperometric gas sensing.Interdigitated finger electrodes on a substrate were used as sensorstructures before but only one or mixed sensing materials weredeposited. The interdigitated finger electrode structure is depositedwith two distinctive sensing materials forming maximum lengththree-point contacts. The sensor was tested continuously for at leastthree weeks at high temperatures showing its robust nature. The sensorstructure could also be used with any other sensing system whichrequires two distinctive deposited materials in an electrochemical cellstructure.

Finally, the sensor is very easy to batch fabricate compared to thebulk-sized sensors and consumes much less power. This is specificallydue to the non-standard photolithographic approach used.

Using the process disclosed herein, sensors may be fabricated which havegood sensitivity, selectivity, response time, and stability.

The carbon dioxide sensor produced by the innovative technique describedherein is applicable to the fire detection (including hidden fire), EVAapplications, personal health monitoring, and environmental monitoring.The sensor and its electronics are integrated into a postage stamp sizedsystem. The low cost due to the batch fabrication process and itscompact size make it highly affordable and thus useable in a wide arrayof locations.

A process for sensing carbon dioxide is accomplished which includes thefollowing steps: applying a constant direct current voltage across thepair of electrodes. The electrodes are separated by an electrolytematerial containing sodium, and the electrodes are located between alayer of alumina substrate and an electrolyte layer of auxiliarycarbonate.

Carbon dioxide is then reacted with the material containing sodium atthe first three-point boundary. The first three point boundary islocated at the joinder of one of the electrodes, the electrolytematerial containing sodium, and the barium containing auxiliaryelectrolyte.

An oxide of sodium is then reacted at a second three-point boundary. Thesecond three-point boundary is located at the joinder of the other ofthe electrodes, the electrolyte material containing sodium, and a bariumcontaining auxiliary electrolyte.

Finally, the resulting current is measured and the change in current iscorrelated to the concentration of carbon dioxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional schematic illustration of a prior art gassensor.

FIG. 1A is a cross-sectional schematic illustration of a prior art bulkgas sensor.

FIG. 2 is a cross-sectional schematic illustration of interdigitatedelectrodes residing on a substrate forming part of the sensor of thepresent invention.

FIG. 2A is a cross-sectional schematic view taken along the lines 2A-2Aof FIG. 2.

FIG. 2B is a cross-sectional view similar to FIG. 2A with first andsecond solid electrolytes over the substrate and the interdigitatedelectrodes.

FIG. 3 is a cross-sectional schematic illustration of a substrate withphotoresist spun onto the substrate.

FIG. 3A is a cross-sectional schematic illustration of the substrate asillustrated in FIG. 3 with a photomask oriented thereover andultraviolet light imidizing the unmasked portions of the photoresist.

FIG. 3B is a cross-sectional schematic illustration of the substrateillustrated in FIG. 3A with the imidized photoresist developed andremoved.

FIG. 3C is a cross-sectional schematic illustration similar to FIG. 3Bwith a first layer of Titanium sputtered onto the substrate.

FIG. 3D is an enlargement of a portion of FIG. 3C illustrating thesputter deposition of the first layer of the Titanium over the substrateand the photoresist.

FIG. 3E is a cross-sectional schematic illustration of a second layer ofPlatinum deposited above the first metallization layer of Titanium andthe photoresist.

FIG. 3F is a cross-sectional schematic illustration of the substratewith two interdigitated electrodes affixed to the substrate with thephotoresist removed with acetone or other suitable solvent.

FIG. 3G is a cross-sectional schematic illustration of photoresist spunover the interdigitated Titanium/Platinum electrodes and the substrate.

FIG. 3H is a cross-sectional schematic illustration of a mask applied tothe substrate and ultra violet light imidizing the unmasked portions ofthe photoresist.

FIG. 3I is a cross-sectional schematic illustration of the substrate,interdigitated electrodes and photoresist left after the imidizedphotoresist has been developed and removed.

FIG. 3J is a cross-sectional schematic illustration of the substrate,interdigitated electrodes with photoresist residing on a portion thereofwith a layer of a first solid electrolyte deposited thereover

FIG. 3K is a cross-sectional schematic illustration wherein thephotoresist has been removed with acetone or other suitable solvent.

FIG. 3L is a cross-sectional schematic illustration with a second solidelectrolyte deposited thereover.

FIG. 3M is a cross-sectional schematic illustration similar to FIG. 3Lwherein the electrodes form tapered surfaces at the place of joinderwith the electrolytes.

FIG. 3N is an enlargement of a portion of FIG. 3M.

FIG. 3O is a cross-sectional schematic similar to FIG. 3J of anotherexample of the application of a first solid applied over the substrate,interdigitated electrodes, photoresist using sputter deposition.

FIG. 3P is a cross-sectional schematic illustration similar to FIG. 3Kwherein the photoresist has been removed with acetone or other suitablesolvent.

FIG. 3Q is a cross-sectional schematic having a second solid electrolyteapplied over the first solid electrolyte and the interdigitatedelectrodes.

FIG. 3R is a cross-sectional schematic illustration wherein a thirdlayer, a metal oxide layer, is applied over the second solidelectrolyte.

FIG. 4 is a schematic illustration similar to FIG. 3H with the maskslightly misaligned.

FIG. 4A is a schematic illustration of the photoresist developed andremoved with photoresist remaining over the interdigitated electrodesbut not centrally located (misaligned).

FIG. 4B is a schematic illustration similar to FIG. 4A with a firstsolid electrolyte deposited thereover.

FIG. 4C is a schematic illustration with the photoresist lifted offthrough dissolution with acetone.

FIG. 4D is a schematic illustration similar to FIG. 4C with a secondsolid electrolyte deposited over the first solid electrolyte and theinterdigitated electrodes.

FIG. 5 is a schematic illustration similar to FIG. 4 with the maskmisaligned above the substrate, interdigitated electrodes, andphotoresist indicating the application of ultraviolet light thereto.

FIG. 5A is a schematic illustration similar to FIG. 5 with the imidizedphotoresist developed and removed leaving a gap filled with photoresistadjacent the electrodes.

FIG. 5B is a schematic illustration with a first electrolyte over thesubstrate, interdigitated electrodes and photoresist.

FIG. 5C is a schematic illustration similar to FIG. 5B with thephotoresist lifted off.

FIG. 5D is a schematic illustration similar to FIG. 5C with a secondelectrolyte over the first electrode and interdigitated electrodes.

FIG. 6 is a schematic illustration of one example of process steps usedto make the sensors.

The drawings will be better understood when reference is made to theDescription of the Invention and Claims which follow hereinbelow.

DESCRIPTION OF THE INVENTION

FIG. 2 is a cross-sectional schematic illustration 200 of interdigitatedelectrodes 204, 210 residing on a substrate 206 forming part of thesensor of the present invention. Positive contact pad 201 isinterconnected by lead 202 to positive bus 203 which is in turninterconnected with positive interdigitated positive electrodes(fingers) 204. Negative contact pad 207 is interconnected by lead 209 tonegative bus 209 which in turn is interconnected with negativeinterdigitated negative electrodes (fingers) 210. Electrodes 204, 210are fixedly engaged to the Alumina substrate 206. The Alumina substrate206 is an insulator and is approximately 625 μm thick.

Still referring to FIG. 2, reference numeral 205 indicates the gapbetween electrodes 204, 210. The electrode width 212W and width of thegap between electrodes 211 are both around 30 μm. See FIG. 2A. Contactpads 201, 207 are interconnected by a conductor 221 to battery 222 whichis nominally at 1V DC. Amp meter 220 measures and records current in thecircuit.

FIG. 2A is a cross-sectional schematic view 200A taken along the lines2A-2A of FIG. 2. Gap 205 and electrodes 204, 210 are illustrated as isthe negative bus 209. In one example illustrated herein, the width 211of the gap 205 is approximately 30 μm. The electrodes 204, 210 have awidth of approximately 30 μm as indicated by reference numeral 212W. Athin layer of Titanium 213 is beneath Platinum electrodes 204, 210.Alternatively, the electrode material may comprise a thin layer ofPtO_(x) followed by a relatively thick layer of Platinum.

FIG. 2B is a cross-sectional schematic view 200B similar to FIG. 2A withfirst and second solid electrolytes 212, 211 over the substrate 206 andinterdigitated electrodes 204, 210. Reference numeral 212 is also usedto indicate the contour of the first electrolyte, for example, NASICON,LISICON, or β″-Alumina (beta prime-prime alumina in which when preparedas an electrolyte is complexed with a mobile ion selected from the groupconsisting of Na⁺, K⁺, Li⁺, Ag⁺, H⁺, Pb²⁺, Sr²⁺, or Ba²⁺). The firstelectrolyte may be any number of solid electrolytes known for theirconductivity performance. These electrolytes may include sodium orlithium as in the case of NASICON and LISICON, but the electrolyte isnot limited to materials containing these elements and may include anynumber of elements including but not limited to Na, Li, K, Ag, H, Pb,Sr, or Ba. The second solid electrolyte 211 may include Sodium Carbonate(Na₂CO₃) or mixture of Sodium Carbonate (Na₂CO₃) and Barium Carbonate(BaCO₃). Other electrolyte materials such as Li₂CO₃, K₂CO₃, Rb₂CO₃,SrCO₃, Ag₂CO₃, PbCO₃ and their mixtures among them or others may be usedas a mixture in place of or in addition to Sodium Carbonate or mixtureof Sodium Carbonate (Na₂CO₃) and Barium Carbonate in a second solidelectrolyte layer.

FIG. 3 is a cross-sectional schematic illustration 300 of a substrate301 with photoresist 302 spun onto the substrate 301. FIG. 3A is across-sectional schematic illustration 300A of the substrate 301 asillustrated in FIG. 3 with a photomask 399 oriented thereover andultraviolet light 305 imidizing the unmasked portions of thephotoresist. The photomask 399 includes apertures 304 and opaqueportions 303.

FIG. 3B is a cross-sectional schematic illustration 300B of thesubstrate 301 illustrated in FIG. 3A with the imidized photoresistdeveloped and removed. The imidized portion of the photoresist is theportion which has been exposed to the ultraviolet light. Unimidizedportions 302A of the photoresist remain on the substrate at this step.

FIG. 3C is a cross-sectional schematic illustration 300C similar to FIG.3B with a first layer of titanium 303 sputtered onto the substrate 301and the unimidized photoresist 302A.

FIG. 3D is an enlargement 300D of a portion of FIG. 3C illustrating thesputter deposition of the first layer of the Titanium 303A over thesubstrate 301 and the photoresist 302A. Titanium layer 303A isapproximately 50 Å thick and forms a good bond to the Alumina substratewhich is approximately 250-624 μm thick. Overall, the dimensions of theinterdigitated area on the Alumina substrate is approximately 1.1 mmlong, 1.0 mm wide and 250 or 625 μm thick in this embodiment of theinvention. FIG. 3E is a cross-sectional schematic illustration 300E of asecond layer of Platinum 304A deposited above the first metallizationlayer of Titanium 303A and the unimidized photoresist 302A.

FIG. 3F is a cross-sectional schematic illustration 300F of the Aluminasubstrate 301 with two interdigitated electrodes 304A/303A affixed tothe substrate with the unimidized photoresist 302A removed with acetoneor some other suitable solvent.

FIG. 3G is a cross-sectional schematic illustration 300G of photoresist355 spun over the interdigitated Titanium/Platinum electrodes 304A/303Aand the Alumina substrate 301. Next, FIG. 3H is a cross-sectionalschematic illustration 300 H of a photomask 399A spaced apart and inproximity to the substrate 301 with interdigitated electrodes 304A/303Athereon and ultra violet light 308 passing through apertures 307imidizing the unmasked (exposed) portions of the photoresist. Referencenumeral 309 represents the width of opaque portion 306 of photomask399A. This width is specifically designed to be less than the width of303A/304A. Once imidization of the photoresist 355 is complete theimidized portions of the photoresist are subjected to developer andremoved leaving the structure in FIG. 3I. FIG. 3I is a cross-sectionalschematic illustration 3001 of the substrate, interdigitated electrodes303A/304A and unimidized photoresist 355A left after the imidizedphotoresist has been developed and removed.

Next, FIG. 3J is a cross-sectional schematic illustration 300J of thesubstrate 301, interdigitated electrodes 304A/303A with unimidizedphotoresist residing on a portion thereof with a layer of first solidelectrolyte 310, for example, NASICON, deposited thereover. Referencenumeral 312 indicates the portion where the NASICON is raised slightlyas its deposition by E-beam evaporation follows the contour of thesubstrate 301, the electrodes 303A/304A and the unimidized photoresist355A. NASICON 310 is applied at a thickness approximately equal to thethickness of the electrodes 303A/304A. E-beam deposition is used here asan example of very controlled, exact deposition of component layersproviding nearly vertical deposition geometries. Actual applications mayvary. In the examples set forth herein (drawing FIGS. 3-3R) one of theelectrodes 303A/304A is the working electrode and the other electrode isthe reference electrode. As indicated in connection with FIGS. 2-2Babove, there may be 8 to 10 pairs of working and reference electrodeswhich combine in an interdigitated fashion to generate enough current toproduce sufficient sensitivity of the sensor. Other numbers of pairs maybe used. The number of electrode pairs used in a sensor depends upon theapplication.

FIG. 3K is a cross-sectional schematic illustration 300K wherein theunimidized photoresist 355A has been removed with acetone, or some othersuitable solvent leaving a contoured surface of NASICON and Platinumelectrodes exposed.

FIG. 3L is a cross-sectional schematic illustration 300L with a secondsolid electrolyte 311 deposited over the NASICON 310 and the electrodes303A/304A. The second solid electrolyte may be Sodium Carbonate(Na₂CO₃), or a combination of Sodium Carbonate (Na₂CO₃) and BariumCarbonate (BaCO₃) thereof in addition to other solid electrolytes andcombinations which may include Li₂CO₃, K₂CO₃, Rb₂CO₃, SrCO₃, Ag₂CO₃, andPbCO₃. The second electrolyte layer with Na₂CO₃ and BaCO₃ mixtureperforms a barrier function in that it keeps the sensor less vulnerableto humidity. Further, it selectively reacts with Carbon Dioxide. at thethree point contacts NASICON 310, the electrode 303A/304A, and theSodium Carbonate or mixture of Sodium Carbonate and Barium Carbonate. Asdescribed elsewhere herein, each electrode joins the NASICON and theSodium Carbonate or mixture of Sodium Carbonate and Barium Carbonatealong a line where reduction and oxidation takes place. Current flowtakes place through the NASICON. Reference numeral 369 representsinboard lines of three point contact of the electrodes 303A/404A,NASICON 312, and second electrolyte Sodium Carbonate 311. Referencenumeral 369A represents outboard lines of three point contact of theelectrodes 303A/404A, NASICON 312, and second electrolyte SodiumCarbonate 311. Depending on the process used for applying the NASICON,the outboard lines 369A may not exist. Such is the case when the NASICONis applied by sputtering as set forth in FIG. 3“O” to FIG. 3R,inclusive.

FIG. 3M is a cross-sectional schematic illustration 300M similar to FIG.3L wherein the NASICON 310, 312 includes tapered surfaces 313 at thejoinder of the Platinum electrodes and the Sodium Carbonate/BariumCarbonate (Na₂CO₃/BaCO₃) layer 311. The tapered surfaces of NASICON arevery thin which in effect creates several lines of multiple three pointcontacts which facilitates the reduction and oxidation processes setforth below. FIG. 3N is an enlargement 300N of a portion of FIG. 3M andprovides a better view of the tapered surface 313, the electrode 304A,first electrolyte 312 and secondary electrolyte 311. It is believed thatthe tapered surface 313 plays an important role in that it provides abetter amperometric surface as the NASICON layer in the tapered surface313 is very thin resulting in multiple lines where three (3) pointcontacts between the electrode, NASICON, and the auxiliary SodiumCarbonate/Barium Carbonate (Na₂CO₃/BaCO₃) electrolyte layer exist.

Still referring to FIG. 3M it is believed that the tapered surfaces 313are created as a result of heat treatment of the NASICON film attemperature as high as 850° C.

The detection system depicted in FIGS. 2-2B and 3-3R includes pairs ofelectrodes with constant voltage, V, applied across the multipleinterdigitated electrodes.

The sensing mechanism of the amperometric CO₂ sensors can be understoodbased on the reactions taking place at the working and referenceelectrode of each pair of electrodes. The following two electrodereactions may be considered:Working Electrode 2Na⁺+CO₂+½O₂+2e ⁻→Na₂CO₃Reference Electrode Na₂O→2Na⁺+½O₂+2e ⁻

Reduction occurs as the result of the reaction taking place at theworking electrode where electrons are consumed. Oxidation occurs as theresult of the reaction taking place at the reference electrode whereelectrons are released.

The following overall reaction can then be considered to be:Overall Reaction Na₂O+CO₂→Na₂CO₃

Platinum is used as the preferred material for the electrode. However,electrodes made from other metals such as Palladium, Silver, Iridium,Gold, Ruthenium, Rhodium, Indium, or Osmium may also be used. Inaddition, non-porous or porous electrodes may be used

The auxiliary electrolyte (Na₂CO₃ and/or BaCO₃ and/or Li₂CO₃, K₂CO₃,Rb₂CO₃, SrCO₃, Ag₂CO₃, PbCO₃) is deposited homogeneously on the entiresensing area of the sensor, including both the working and referenceelectrodes. The deposition of an auxiliary carbonate electrolyteimproves flow of the desired species within the electrolyte. At theworking electrode, depleted concentration of sodium ions (Na⁺) can berecovered by the transfer of sodium ions (Na⁺) from NASICON through thethree-phase boundary of the electrodes, NASICON electrolyte, and anauxiliary electrolyte layer. The sodium carbonate, Na₂CO₃, deposited atthe working electrode can be transferred to the reference electrodethrough the Na₂CO₃ auxiliary carbonate electrolyte if temperatures arehigh enough, for example, 450-600° C.

These mechanisms allow the sensor to measure CO₂ but recover back to itsinitial state. The sensing mechanism has increased performance from theNa₂CO₃/BaCO₃ auxiliary carbonate electrolyte layer being distributedacross both the working and the reference electrodes at high operatingtemperatures in the 450-600° C. The eutectic mixture of Na₂CO₃/BaCO₃ asthe auxiliary carbonate electrolyte layer has a lower meltingtemperature enabling improved flow within the electrolyte at a reducedtemperature range. The Na₂CO₃/BaCO₃ auxiliary carbonate electrolyte canact as a diffusion barrier to prevent other species from reaching theelectrode/electrolyte interface and interfering with the correlation ofmeasured current with detection of the desired chemical species.

FIG. 3 “O” is a cross-sectional schematic 300 “O” similar to FIG. 3J andis another example of the application of a first solid electrolyte 310Aapplied over the substrate 301, interdigitated electrodes 303A/304A andunimidized photoresist 305A using sputter deposition. Sputter depositionof NASICON 310A results in a surface 320 which is contoured and does notfollow the underlying components. Sputter deposition is used here as anexample of a less exact, more diffuse deposition of component layersproviding more graded deposition geometries. Actual applications mayvary. In FIG. 3J, the NASICON was applied in a manner which results inthe NASICON applied so as to more closely follow the contour of theunderlying structure. Reference numeral 325 is used to indicate theNASICON above the unimidized photoresist 305A.

FIG. 3P is a cross-sectional schematic illustration 300P similar to FIG.3K wherein the unimidized photoresist 305A has been removed with acetoneor other suitable solvent leaving NASICON 310A behind with a contouredsurface 320. Next, the auxiliary Sodium Carbonate/Barium Carbonate(Na₂CO₃/BaCO₃) electrolyte composition is applied. FIG. 3Q is across-sectional schematic 300Q having a second solid electrolyte 311applied over the first solid electrolyte 310A and the interdigitatedelectrodes 303A/304A.

FIG. 3R is a cross-sectional schematic illustration 300R wherein a solidmetal oxide 330, SnO₂, CuO, In₂O₃, and TiO₂ and/or a combinationthereof, is applied over the second solid electrolyte 311. It ispreferred that these solid metal oxides be composed of nanoparticles.Use of this third layer of metal oxide provides enhanced performance ofthe sensor. This third layer of metal oxide is applied by dropdeposition of SnO₂ sol gel on top of the Na₂CO₃/BaCO₃ and heat treat thesensor in the instant invention. It can also be deposited using e-beamevaporation or sputtering using a shadow mask which is the same as thatfor Na₂CO₃/BaCO₃ deposition. The third layer of metal oxide improves thesensor signal greatly and also enables the carbon dioxide sensor tofunction a temperature range as low as 200° C.

FIG. 4 is a schematic illustration 400 similar to FIG. 3H with thephotomask 499A slightly misaligned. FIGS. 4-4D are illustrative of thefault tolerance of the instant invention. It is this fault tolerancewhich enables successful production of the device. The opaque portion404 of the mask 499A is indicated with reference numeral 404 and theultra violet light 405 is indicated with reference numeral 405. Stillreferring to FIG. 4, it can be seen as is discussed elsewhere hereinthat if the opaque portions 404 of the photomask 499A are the same widthas the underlying electrodes 402/406 and they are misaligned, then afaulty sensor will result. For this reason it is necessary that theopaque portions of the mask have a width less than 30 μm and preferablyin the range of 15-20 μm. Misalignment of the photomask 499A(serpentine) results in decentralized unimidized photoresist 403A.However, because the opaque portions of the mask have a widthsignificantly smaller than the width of the electrodes perfect alignmentis not necessary. The NASICON lip shown by reference numeral 412 and theNASICON indicated by reference numeral 313 are locations where theNASICON may be thin and in effect leads to more reaction sites close tothree boundary contacts. Also, this speeds up the manufacturing processbecause the technician does not have to be perfect in alignment. This isin contrast to standard industry practice which emphasizes increasingtight alignment and deposition procedures; the approach here is to allowand in fact take advantage of diffuse deposition and inexact alignmentsto improve the sensor response. In the example of FIG. 4, referencenumeral 406 is the thin layer (50 Å) of Titanium as previously describedin connection with reference numeral 303A in FIGS. 3-3R. Referencenumeral 402 is the relatively thicker layer (4000 Å) of Platinum aspreviously described in connection with reference numeral 304A in FIGS.3-3R.

FIG. 4A is a schematic illustration 400A of the photoresist developedand removed with unimidized photoresist 403A remaining over theinterdigitated electrodes but not centrally located (misaligned). FIG.4B is a schematic illustration similar 400B to FIG. 4A with a firstsolid electrolyte such as NASICON or LISICON 410 deposited thereover bye-beam evaporation. FIG. 4C is a schematic illustration 400C with thephotoresist lifted off through dissolution with acetone or othersuitable solvent. Raised portions of the NASICON or LISICON 410 areviewed well in FIG. 4C. FIG. 4D is a schematic illustration 400D similarto FIG. 4C with a second solid electrolyte 411 (Barium Carbonate and/orSodium Carbonate) deposited over the first solid electrolyte and theinterdigitated electrodes. FIG. 4D illustrates the potential problemwith misalignment discussed elsewhere herein particularly in describingFIG. 1 which can not be produced because of the stack-up ofmanufacturing tolerances.

FIG. 5 is a schematic illustration 500 similar to FIG. 4 with thephotomask 599A significantly misaligned above the substrate 501,interdigitated electrodes 503/504, and photoresist 555 indicating theapplication of ultraviolet 508 light thereto. Reference numeral 503 isthe Titanium layer of the electrode and reference numeral 504 is thePlatinum layer of the electrode as described and similarly proportionedto the other examples given herein. Opaque portions of the photomask599A are indicated by reference numeral 506 and apertures in the maskare denoted by reference numeral 507. Misalignment should not occur whenthe opaque portions 506 of the photomask 599A are substantially smallerthan the width of the electrodes as described herein. However, theillustration of FIG. 5 is being made to demonstrate that a problem ismore likely to occur when the mask width equals the width of theelectrodes as is the standard industry practice and direction. As thewidth of the opaque portion of the photomask increases or approximatesthe width of the electrode, the probability of misalignment increases.As was the case of the examples illustrated in FIGS. 3-3R and 4-4D, theopaque portion 506 of the mask protects the underlying photoresist andprevents ultraviolet light from reaching the photoresist resulting in aportion of the photoresist being unimidized 555A.

FIG. 5A is a schematic illustration 500A similar to FIG. 5 with theimidized photoresist developed and removed leaving a gap filled withunimidized photoresist 555A appearing just to the left of the electrodes503/504. This photoresist 555A which lies next to the electrodes 503/504will interfere with the proper function of the electrodes as it preventsthe joinder of the electrodes, NASICON, and the Carbonate layer asillustrated in FIG. 5D. It also blocks the movement of Na ion in NASICONbetween reference electrode and working electrode, which is also acritical factor for sensor to work or function.

FIG. 5B is a schematic illustration 500B with a first electrolyteNASICON deposited by e-beam deposition over the substrate,interdigitated electrodes, and photoresist. It will be noticed that theNASICON 510 does not abut the electrodes 503/504 on the left hand sideof FIG. 5B. FIG. 5C is a schematic illustration 500C similar to FIG. 5Bwith the unimidized photoresist 555A lifted off with acetone. NASICON510 includes a raised portion 512. FIG. 5D is a schematic illustration500D similar to FIG. 5C with a second electrolyte 511 over the firstelectrolyte and the interdigitated electrodes 503/504.

In describing the success or failure of the carbon dioxide sensor theelectrodes are interdigitated and may involve 8-10 pairs of electrodesin order to sum enough current to provide the desired sensitivity.Currents ranging from nano to micro amps are generated by theapplication of 1.0 Volts or higher dc across the sensor electrode bus asillustrated schematically in FIG. 2.

The fabrication of carbon dioxide sensors includes three steps: 1)Deposition of platinum interdigitated finger electrodes on Aluminasubstrates; 2) Deposition of solid electrolyte called NASICON(Na₃Zr₂Si₂PO₁₂) or LISICON (Li₃Zr₂Si₂PO₁₂) between the fingerelectrodes; and 3) Deposition of auxiliary electrolytes sodium carbonateand/or barium carbonate (Na₂CO₃/BaCO₃, 1:1.7 in molar ratio for thecombination) on the upper surfaces of the electrodes.

The Platinum interdigitated finger electrodes were deposited as follows:Alumina substrates (250 μm or 625 μm in thickness) were patterned withphotoresist and an interdigitated finger electrode photomask. A 50 Ålayer of Titanium and a 4000 Å layer of Platinum were deposited on theAlumina substrate by sputter deposition. After development and removal,the substrates were then patterned again to cover the top ofinterdigitated finger electrodes with photoresist.

Deposition of the NASICON solid electrolyte between the fingerelectrodes and the Na₂CO₃/BaCO₃ was performed as follows. The solidelectrolyte NASICON was deposited by e-beam evaporation or sputtering. Aliftoff process which uses acetone to remove unimidized photoresist wasconducted to remove NASICON on the upper surfaces of the electrodesresulting in the NASICON mainly staying between the interdigitatedfinger electrodes and exposing most of the electrode surface. Thesubstrate was heated in an oven at 850° C. for 2 hours. Na₂CO₃/BaCO₃(1:1.7 in molar ratio) was then deposited on the upper surfaces of theelectrodes and the NASICON surface by sputtering using a shadow mask.The use of shadow mask in this step is to prevent the Na ion indeposited NASION being washed away by photolithograph process, which isnot obvious and not a typical practice of standard microfabricationprocess. The substrates were heated in an oven at 686° C. for 10 minutesand 710° C. for 20 minutes. Different concentrations of carbon dioxidegases were tested by the sensors at temperatures ranging from 450-600°C. The sensor was tested by applying a voltage to the electrodes andmeasuring the resulting current. A linear response to carbon dioxideconcentrations between 1% to 4% was achieved. Linear responses of thenatural logarithmic of carbon dioxide concentrations between 0.02% to 1%was achieved.

The resulting miniature CO₂ sensor can be integrated into a sensor arraywith other sensors and electronics, power, and telemetry on a stampsized package. Like a postage stamp, the complete system (“lick andstick” technology) can be placed at a number of locations including somehidden areas to give a full-field understanding of what is occurring inan environment. The same sensor structure could also be applied todevelop NO_(x) or SO_(x) with the corresponding auxiliary electrolytesNaNO₂ and NaNO₃, or Na₂CO₃ and Na₂SO₄.

FIG. 6 is a schematic illustration 600 of one example of process stepsused to make the sensors. The process steps are described below and havebeen described hereinabove.

First, an Alumina substrate is coated with photoresist 302. A photomask399 is then applied selectively 602 imidizing ultra violet light usingan interdigitated finger electrode photomask and developing and removingthe imidized photoresist. Next, sputtering 603, a 50 Å layer of Titanium303A onto the Alumina 301 substrate and unimidized photoresist 302A isperformed. The sputtering of the Titanium is followed by sputtering 604a 4000 Å layer of Platinum onto the Titanium.

The unimidized photoresist 302A is lifted off 605 with acetone or othersolvent to remove the unimidized photoresist 302A as well as theTitanium 303A and Platinum 304A thereover forming electrodes on theAlumina substrate. Another layer of photoresist is then applied 606 tothe Alumina substrate 301 and electrodes 303A/304A. The photoresist isselectively imidized 607 by applying imidizing ultraviolet light 308using an interdigitated finger electrode photomask 399A and thendeveloping and removing the imidized photoresist. Electron beamevaporation or sputtering 608 of NASICON over the Alumina substrate, theelectrodes and the unimidized photoresist follows. Lifting off 609 theunimidized photoresist and NASICON thereover with acetone or othersolvent is then performed so as to enable the deposition of secondaryelectrolyte 610 using a shadow mask over the NASICON and the electrodes.The step 620 of depositing a metal oxide may be accomplished by dropdeposition of metal oxide sol gel or by sputtering/e-beam depositionusing a shadow mask

REFERENCE NUMERALS

-   100—schematic of prior art device-   100A—schematic of prior art device-   101—NASICON-   102, 110—Sodium Carbonate/Barium Carbonate (Na₂CO₃/BaCO₃)-   103—Platinum paste-   104—sensing electrode-   105—reference electrode-   106—quartz glass tube-   107—Alumina-   108—interdigitated Platinum metal electrodes-   109—NASICON-   200—schematic of interdigitated metal electrodes-   200A—schematic view of section 2A-2A-   200B—view of section 2A-2A with NASICON and Barium Carbonate/Sodium    Carbonate thereover-   201—contact pad-   202—lead-   203—positive bus-   204—interdigitated positive metal electrodes-   205—gap between electrodes-   206, 301—substrate (insulator)-   207—contact pad-   208—lead-   209—negative bus-   210—interdigitated negative metal electrodes-   211—width of gap between electrodes-   212—contour of NASICON after liftoff of photoresist-   212W—width of electrode-   213—thin layer of Titanium metal-   220—amp meter-   221—conductor-   222—battery or electrical potential-   300—schematic view of substrate with photoresist spun thereover-   300A—schematic view of mask over substrate with photoresist spun    thereover-   300B—schematic view of substrate with imidized photoresist developed    and removed-   300C—schematic view of substrate and unimidized photoresist with a    thin layer of Titanium thereover-   300D—enlargement of a portion of FIG. 3C-   300E—schematic view of second metal layer of Platinum applied over    the first metal layer of Titanium-   300E—schematic view of interdigitated electrodes and substrate after    liftoff of photoresist-   300G—schematic view of photoresist spun over the interdigitated    electrodes and substrate-   300H—schematic view of mask placed over photoresist-   300I—schematic view of substrate, electrodes and unimidized    photoresist after the imidized photoresist has been developed and    removed-   300J—schematic view of NASICON deposited by e-beam evaporation over    the substrate, electrodes, and photoresist-   300K—schematic view with the photoresist lifted off-   300L—schematic view similar to FIG. 3K with a second electrolyte    deposited over the NASICON and electrodes-   300M—schematic view of another example of the invention wherein    multiple three point contacts occur between the NASICON, the    electrodes and the second electrolyte-   300N—schematic view of an enlargement of a portion of FIG. 3M-   300 “O”—schematic view similar to FIG. 3J wherein NASICON is    sputtered over the substrate, electrode and the photoresist-   300P—schematic view similar to FIG. 3K wherein the unimidized    photoresist has been lifted off-   300Q—schematic view a second electrolyte sputtered, using a shadow    mask, over the NASICON and electrodes-   300R—schematic view of a third electrolyte sputtered, using a shadow    mask, over the second electrolyte-   301, 401, 501—Alumina, substrate (non-conductive)-   302, 355, 403, 555—photoresist-   302A, 305A, 355A, 403A, 555A—unimidized photoresist-   305, 308, 405, 508—UV light-   303, 306, 404, 506—opaques portions of photomask-   303A, 406, 503—thin first Titanium metal layer-   304, 307, 507—aperture in mask-   304A, 402, 504—second Platinum metal layer-   305, 308—ultraviolet light-   309—width of opaque portion of mask-   310, 410, 510—NASICON, first solid electrolyte, e-beam deposited-   310A—NASICON, first solid electrolyte, sputter deposited-   311, 411, 511—second solid electrolyte Sodium Carbonate/Barium    Carbonate (Na₂CO₃/BaCO₃)-   312, 412, 512—raised portion of NASICON-   313—extended three-point contact-   320—contour of sputter deposited NASICON-   325—sputter deposited NASICON-   330—layer of metal oxide, SnO₂, CuO, and TiO₂.-   369—inboard three point electrical contact-   369A—outboard three point electrical contact-   399, 399A, 499A, 599A—photomask-   400—schematic view similar to FIG. 3H with the mask slightly    misaligned although still over the electrodes electrolyte is sputter    deposited over the NASICON and electrodes-   400A—schematic view similar to FIG. 3I with the imidized photoresist    developed and removed-   400B—schematic view similar to FIG. 3J with NASICON deposited by    e-beam evaporation over the substrate, electrodes and unimidized    photoresist-   400C—schematic view similar to FIG. 3K with the unimidized    Photoresist lifted off-   400D—schematic view similar to FIG. 3L wherein a second-   500—schematic view similar to FIG. 4 with the mask misaligned-   500A—schematic view similar to FIG. 4A with the unimidized    photoresist extending beyond the electrodes-   500B—schematic view similar to FIG. 4B with NASICON deposited over    the substrate, unimidized photoresist and electrodes.-   500C—schematic view similar to FIG. 4C with the unimidized    photoresist developed and removed.-   500D—schematic view similar to FIG. 4D with a second solid    electrolyte deposited over the NASICON, unimidized photoresist and    metal electrodes-   600—one example of process steps used to fabricate the sensor-   601—applying photoresist on Alumina substrate-   602—applying, selectively, imidizing ultra violet light using an    interdigitated finger electrode photomask, developing and removing    the imidized photoresist-   603—sputtering a 50 Å layer of Titanium onto the Alumina substrate    and unimidized photoresist-   604—sputtering a 4000 Å layer of Platinum onto the Titanium-   605—lifting off with acetone or other solvent the unimidized    photoresist, Titanium and Platinum thereover forming electrodes on    the Alumina substrate-   606—applying photoresist to the Alumina substrate and electrodes-   607—applying, selectively, imidizing ultraviolet light using an    interdigitated finger electrode photomask, developing and removing    the imidized photoresist-   608—electron beam sputtering of NASICON over the Alumina substrate,    the electrodes and the unimidized photoresist-   609—lifting off with acetone or other solvent the unimidized    photoresist and NASICON thereover-   610—depositing secondary electrolyte using a shadow mask over the    NASICON and the electrodes-   620—depositing metal oxide using metal oxide sol gel or by    sputtering/e-beam deposition

The invention has been set forth by way of example. Those skilled in theart will recognize that changes may be made to the invention withoutdeparting from the spirit and the scope of the claims which followherein below.

1. A gas sensor comprising: a substrate; a pair of interdigitated metalelectrodes selected from the group consisting of Pt, Pd, Au, Ir, Ag, Ru,Rh, In, and Os; said electrodes each include an upper surface; a firstsolid electrolyte resides between said interdigitated electrodes andpartially engages said upper surfaces of said electrodes; said firstsolid electrolyte selected from the group consisting of NASICON,LISICON, KSICON, and β″-Alumina and, a second electrolyte partiallyengaging said upper surfaces of said electrodes and engaging said firstsolid electrolyte in at least one point, said second electrolytecomprising an element selected from the group consisting of Na⁺, K⁺,Li⁺, Ag⁺, H⁺, Pb²⁺, Sr²⁺, and Ba²⁺ ions or combinations thereof.
 2. Agas sensor as claimed in claim 1 wherein said substrate is selected fromthe group consisting of Al₂O₃ or fused SiO₂ or other insulator.
 3. A gassensor as claimed in claim 1 wherein said electrodes are porous.
 4. Agas sensor as claimed in claim 1 wherein said electrodes are non-porous.5. A gas sensor as claimed in claim 1 wherein said electrodes are metalalloyed with another metal selected from the group consisting of Pt, Pd,Au, Ir, Ag, Ru, Rh, In, and Os.
 6. A gas sensor as claimed in claim 1further comprising another layer and wherein said another electrolyticlayer is one of the following SnO₂, TiO₂, In₂O₃, CuO, ZnO, Fe₂O₃, ITO orother metal oxide or any mixtures of the preceding compounds.
 7. A gassensor as claimed in claim 1 wherein said electrodes can be deposited bysputter or evaporation, the solid electrolyte can be deposited bysputter or evaporation, and the auxiliary electrolyte(s) can bedeposited by using a shadow mask.
 8. A gas sensor as claimed in claim 7further comprising a layer of metal oxide selected from the groupconsisting SnO₂, TiO₂, In₂O₃, CuO, ZnO, Fe₂O₃, ITO or other metal oxideand their mixtures residing above and in engagement with said secondelectrolyte.
 9. A gas sensor as claimed in claim 1 wherein an electricpotential is applied between said electrodes and current output ismeasured to determine the gas concentration.
 10. An electrochemical cellcomprising: an interdigitated electrode selected from the groupconsisting of Pt, Pd, Au, Ir, Ag, Ru, Rh, In, Os, Cr, and Ti; each ofsaid interdigitated electrodes include an upper surface and a lowersurface; a substrate layer; said lower surface of said interdigitatedelectrodes deposited onto said substrate layer and secured thereto; and,a first solid electrolyte residing between and in engagement with saidinterdigitated electrodes and engaging said upper surfaces of saidinterdigitated electrodes; said first solid electrolyte selected fromNASICON, LISICON, and β″-Alumina; and a layer of metal carbonate(s) asan auxiliary electrolyte engaging said upper surfaces of said electrodesand said first solid electrolyte; said metal carbonate comprising anelement selected from the group consisting of the following ions Na⁺,K⁺, Li⁺, Ag⁺, H⁺, Pb²⁺, Sr²⁺, Ba²⁺, and any combinations thereof.
 11. Anelectrochemical cell as claimed in 10 wherein an electric potential isapplied between the working and reference electrodes and current outputis measured to determine the gas concentration.
 12. A gas sensor asclaimed in claim 10 further comprising an extra layer of metal oxideselected from the group consisting of SnO₂, TiO₂, In₂O₃, CuO, ZnO,Fe₂O₃, ITO or other metal oxide residing above and in engagement withsaid second electrolyte.
 13. A gas sensor comprising: a substrate layer;a pair of interdigitated metal electrodes, said electrodes include uppersurfaces, said electrodes, selected from the group consisting of Pt, Pd,Au, Ir, Ag, Ru, Rh, In, Os, and their alloy; a first layer of solidelectrolyte staying in between electrode fingers and partially on saidupper surfaces of said electrodes, said first layer selected fromNASICON, LISICON, KSICON and β″-Alumina; a second layer of metalcarbonate(s) auxiliary as an electrolyte engaging said upper surfaces ofsaid electrodes and said first solid electrolyte; said metal carbonatescomprising an element selected from the group consisting of thefollowing ions Na⁺, K⁺, Li⁺, Ag⁺, H⁺, Pb²⁺, Sr²⁺, Ba²⁺, and anycombination thereof; and an extra layer of metal oxide selected from thegroup consisting of SnO₂, TiO₂, In₂O₃, CuO, ZnO, Fe₂O₃, ITO or othermetal oxide and their mixtures residing above and in engagement withsaid second electrolyte.